Article 2, Section 1, Clause 7

The President shall, at stated Times, receive for his Services, a Compensation, which shall neither be encreased nor diminished during the Period for which he shall have been elected, and he shall not receive within that Period any other Emolument from the United States, or any of them.

1.Massachusetts Constitution of 1780, PT. 2, CH. 2, SEC. 1, ART. 13
2.Records of the Federal Convention
3.Alexander Hamilton, Federalist, no. 73, 492--94, 21 Mar. 1788
4.House of Representatives, Compensation of President, 16 July 1789